Friday, September 26, 2014

AVR ISP Pogo adapter

I've been buying and using SparkFun's ISP Pogo adapter, but there are a couple of things about it's design that - at least for me - could be improved.

The basic design - two boards that are rigidly fixed in a precise vertical alignment each supporting the pins - is good. But I don't value the Molex and JST connectors. I'd rather be able to grasp more easily onto the top board without having so much stuff in the way.

Fortunately, designing one isn't hard. Just put two 6 pin AVR ISP footprints on a board and connect each related trace. You use a pair of boards to make one fixture, and since they're both the same, they'll both align perfectly. You use a pair of 1/4" #4 unthreaded spacer between the two boards and a 1/2" #4 bolt and #4 nut to hold the boards together. With the spacers, there will be enough room on the lower board to install a right-angle DIP header, leaving the top of the top board completely open and usable as a grip/pressure surface.

You assemble the adapter and align the pogo pins the same way SparkFun says to do it on theirs. Start  by attaching a 2x3 right angle DIP header to what will become the bottom board, making sure to install it on the TOP of the board, and that it doesn't stick up more than 1/4 inch (so that it won't touch the top board once it's in place). Next, mechanically assemble the two boards together with the two machine screws, spacers and nuts, making sure that the TOP of each faces the same direction and is oriented the same way. Next, temporarily attach a piece of cellophane tape to the top of the top board, then insert the pogo pins through the bottom board and into the holes of the top board so that they rest against the tape. Carefully check the pins to make sure they're straight and level. If the pins are racked, twist the boards so that they're straight, loosening the hardware if necessary. Solder the pins to the bottom board, then remove the tape and solder the pins from the top of the top board. The second footprint on the top board will be left unconnected.

It turns out that SparkFun's drill holes are the same diameter as the Pogo pin itself. They may be counting on their PCB vendor consistently erring on larger holes. I can only hope that OSHPark does the same. A snug fit is ok, but not fitting at all would be bad. We'll see.

I envision bigger things from this design, potentially. Because the top is (nominally) flat, you could remove the top two screws and screw longer ones in, attaching the whole adapter to a larger programming jig, possibly for mass programming panels of devices all at once. The only hard part of that is figuring out how to get avrdude to individually address multiple USBTiny programmers simultaneously attached.

4 comments:

  1. What do you think about using these: http://www.digikey.com/short/tj205t - instead of pogo pins.
    I ordered ED8179-06-ND but they do have longer legs if needed.
    And yes, they are more expensive, but they are sturdier.

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    1. It's difficult to tell from the renders on DigiKey. I do see that "spring" is in the item description. The standard footprint I use for pogo pin programming consists of 4 SMD pads and 2 through-holes. The holes help line the pogo pins up. It would be interesting to see how these work with that.

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  3. The one I ordered will actually go thru the holes in an ISP socket (non-populated) while providing a good contact surface without having to rely on the spring force to keep them on the board.

    For example:
    https://www.dropbox.com/sc/zwy7mwjtc8dho2o/AADA8HJRKNTMSNpIA8wPZ8qXa

    https://www.dropbox.com/sc/lmmxhbxyghj760m/AABwdhMAogTvPtiwyLzxZ9lOa

    I will probably order the SMD version in the future just to see how much spring action is there, but with the way it fits currently, as long as the leg width and length are good, I don't see too many issues with them for this application.

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